%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-3-2 on page II-155 in "amsmath, mathtools --- The matrix environments".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{207.0pt}

%StartShownPreambleCommands
\usepackage{amsmath}
\setcounter{MaxMatrixCols}{20}
%StopShownPreambleCommands

\begin{document}
\[
 \begin{Vmatrix}
  \,a&b&c&d&e&f&g&h&i&j &\cdots\,{} \\
     &a&b&c&d&e&f&g&h&i &\cdots\,{} \\
     & &a&b&c&d&e&f&g&h &\cdots\,{} \\
     & & &a&b&c&d&e&f&g &\cdots\,{} \\
     & & & &\ddots&\ddots&\hdotsfor[2]{5}\,{}
 \end{Vmatrix} \]
\end{document}
